1. Field of the Invention
The present invention relates to a printing apparatus and a printing method, particularly to a printing apparatus which prints an image by discharging ink onto a print medium while reciprocally moving a carriage which mounts an inkjet printhead, and a printing method used by the apparatus.
2. Description of the Related Art
In image printing by an inkjet printing apparatus (to be referred to as a printing apparatus hereinafter), a high-quality image cannot be obtained unless a plurality of ink droplets which form the image land at correct positions on a print medium (for example, a printing sheet) to form dots in a relatively correct array. However, variations occur in landing position of dots due to various errors generated in the printing apparatus.
In a printing apparatus which prints on a print medium while scanning a printhead which is mounted to a carriage, the scanning direction of the carriage must be perpendicular to the nozzle array direction of the printhead. However, due to factors associated with, for example, the nozzle manufacturing accuracy of the printhead, and a tolerance upon mounting the printhead on the carriage, the nozzle array direction may not be perpendicular to the carriage scanning direction. If the perpendicular relationship cannot be obtained, the actual nozzle array direction with respect to the expected direction will be referred to as a head slant hereinafter.
FIG. 13 shows views of the relationship between the head slant and the formed image.
b in FIG. 13 shows the case wherein no head slant is present, a in FIG. 13 shows the case wherein a head slant is in the obliquely lower left direction, and c in FIG. 13 shows the case wherein a head slant is in the obliquely lower right direction. As can be seen from comparisons among these three drawings, even when printing is similarly done on a printing sheet 3 by a nozzle array 10 of a printhead 9 (the vertical ruled line in this example), printing is normally done in the absence of a head slant, but printing is not correctly done in the presence of a head slant. Such a slant derives from, for example, a shift between the nozzle array direction and the carriage scanning direction upon mounting the printhead on the carriage, or bending of a main rail 8 provided for carriage scanning.
A method of setting an address management unit for reading out printing data, corresponding to the head slant, according to this slant, and setting the value of the head slant to be corrected for the address management unit has been conventionally proposed (see, for example, Japanese Patent Laid-Open No. 2004-009489).
FIG. 14 shows views of the concept of slant correction according to the conventional technique.
d in FIG. 14 shows printing data stored in a print buffer, and e in FIG. 14 is a schematic view showing the timing at which slant correction is performed to print on a print medium. An example in which slant correction is done at positions corresponding to printing data on columns a, f, and j stored in the print buffer will be described with reference to FIG. 14. FIG. 14 shows an example where slant correction of 2 columns is performed for column a, slant correction of 1 column is performed for column f, and slant correction is not performed for column j. In conventional slant correction, as shown in e of FIG. 14, printing data on column a is read from the print buffer so as to be printed at a printing timing corresponding to three columns A to C. Also, printing data on column f is read from the print buffer so as to be printed at a printing timing corresponding to 2 columns F and G, and that on column j is read from the print buffer so as to be printed at a printing timing corresponding to 1 (one) column J.
With this operation, by printing while changing a timing at which printing data is read from the print buffer in accordance with the head slant, the slant of printing dots formed on the print medium is corrected, thus allowing printing free from any slant.
However, in the case of slant correction in the above-mentioned conventional example, when the timing of reading from the print buffer is changed for each carriage position, data may not be completely read, resulting in local loss of an image.